How do you work with low-res footage and render with high-res footage in the video editor?
The internal proxy generation is very slow in Blender, but what you do is to add all of your clips to the timeline and select them all(A).

Go to the sidebar, hit Set Selected Strip Proxies.

Choose your Proxy resolution.

Hit Rebuild Proxy. 

Wait.

In Preview open the sidebar, in the View Tab, select the Proxy resolution.
(the proxies will stay even though you remove all of your footage again)

Or use this add-on for faster batch rendering of proxies: 
blendervelvets.org/en/velvet-revolver